1. Field of the Invention
The present invention involves the monitoring of growth rate and composition of an epitaxial layer being fabricated from a plurality of atomic species in a vacuum chamber.
Many widely known deposition techniques such as molecular beam epitaxy (MBE) and electron-beam evaporation are used in the fabrication of many types of devices including electronic and opto-electronic devices. An important need exists to improve both the accuracy and the yield of high performance electronic and opto-electronic device fabrication. Some examples of these devices are Resonant Tunnel Diodes (RTD), Vertical Cavity Surface Emitting Lasers (VCSEL), electro-optical modulators, and quantum well lasers. Often, the process for making these device involves growing thousands of distinct epitaxial layers while maintaining strict control of the composition and thickness of each layer. In addition, there are further constraints such as:
1) limited access of the monitoring system to the vacuum growth chamber. PA1 2) monitoring system has to be non-invasive to the growth process. PA1 3) error due to noise and drift has to be sufficiently small during the course of the growth. PA1 4) monitoring system must have a high rejection of stray light from the surrounding environment. PA1 5) monitoring system must not be affected by special requirements such as sample rotation during MBE growth. PA1 6) the monitoring system should be portable and most of the setup should be remote from the growth chamber.
Currently, many epitaxial deposition techniques such as electron-beam evaporation and Molecular Beam Epitaxy (MBE) lack good growth monitoring systems which have the ability to monitor in real-time the growth rate as well as composition during multi-component epitaxial growth. Included among these monitoring systems is the technique of atomic absorption.
It has long been recognized that atomic absorption can be used as a tool for monitoring and control of material deposition processes. Atomic absorption techniques generally involve passing a light beam through the molecular beam of the MBE process and then measuring the resulting intensity of the light beam. The more atoms that are being deposited, the greater the atomic absorption, resulting in a lower intensity of light. The light beam must have a wavelength that corresponds with the atomic species desired to be measured. In this way the growth of the epitaxial layers with respect to a particular atomic species can be measured. Recently, because of a need for better non-invasive real-time feedback during the growth process, increased attention has been turned toward monitoring techniques using atomic absorption of the molecular beam flux.
2. Description of the Related Art
There have been many prior monitoring systems which utilize the atomic absorption technique. However, the prior art discloses the monitoring of just one channel at a time with the possibility of combining several independent one-channel units to build a multi-channel system. These multi-channel systems do not integrate the channels and therefore require additional port space in the vacuum chamber.
Appl. Phys. Lett. 60 (5) Feb. 3, 1992, p. 657, (Klausmeier et al.), discloses the passing of a modulated light beam through the molecular beam of an MBE process and then passing the light beam through a bandpass filter and into a photomultiplier tube. The filter passes only a particular emission line so that only the growth of the atomic beam flux corresponding to that particular wavelength is measured. If more than one atomic beam flux is being deposited on the substrate then the Klausmeier device can only measure the growth rate and composition of one atomic species.
J.Vac.Sci.Technol.B 12(2), March/April 1994, p. 217, and J. Vac. Sci. Technol. A 13(3), May/June 1995, p. 1797, disclose a system similar to Klausmeier's and the implementation of multiple channels is not directly addressed.
Appl. Phys. Lett. 63 (23), Dec. 6, 1993, p. 3131, and Appl. Phys. Lett. 65 (1), Jul. 4, 1994, p. 4, disclose a dual-beam configuration with two channels but it is not optimized for the limited optical access that exists in most MBE and electron beam evaporation systems. The two channels are not integrated to follow one path through the molecular beam and therefore require more port space to the vacuum chamber.